1. Field of the Invention
The present invention relates to an image sensor for use in, for example, an image capture apparatus.
2. Description of the Related Art
Recently, in an image sensor for use in an image capture apparatus, such as an electronic camera, has been required to have high sensitivity and high pixel density, which are conflicting features. Further, expanding a condition of a permissible incident angle at an image-plane peripheral portion (where an image height is large) has also been required.
To achieve such a high functional electronic camera, high sensitivity of the image sensor is required so that a shutter can be operated at high speed even under a small light quantity to capture an image with less shake. To realize high sensitivity, it is necessary to efficiently convert the equivalent amount of light reaching the image sensor into an electric signal with lower noise.
On the other hand, to achieve a higher image quality, high pixel density for capturing a high-definition image is being promoted by increasing the number of pixels of the image sensor. Since the size of the image sensor is limited, increasing the number of pixels is achieved by decreasing a pitch between pixels on the image sensor. Accordingly, in order to realize high pixel density, an area size per pixel needs to be reduced.
Recently, the pixel pitch has been already smaller than the diameter of a minimum light spot which can be focused by an imaging lens, depending on an actually-used aperture value. Generally, as an area size per pixel is reduced, the sensitivity of the image sensor deteriorates. A quantity of received light is decreased due to the small pixel area. Further, the area of a photoelectric conversion element (generally called a photodiode, or referred to as a PD), which substantially converts light into electricity in the image sensor, is relatively reduced. The area of the photoelectric conversion element per pixel varies according to the configuration of a sensor such as a charge-coupled device (CCD) sensor or a complementary metal oxide semiconductor (CMOS) sensor. However, generally, the area of the photoelectric conversion element is smaller than the size of one pixel, and only part of a light quantity, which is incident to the area for one pixel, contributes to photoelectric conversion. One means for achieving high sensitivity is to increase the contribution ratio. Since most of light flux incident to the area of one pixel determined by the pixel pitch is propagated in various directions and lost before the light flux is incident from the surface of the image sensor to the inside thereof to reach the photoelectric conversion element, it is not easy to achieve the high sensitivity.
To achieve high sensitivity, in the related art, there has been generally used a method of installing a micro lens (on-chip micro lens, and hereinafter, referred to as a microlens) as a two-dimensional microlens array on the surface of every pixel. The light flux incident to the entire pixel area is further collected in the image sensor by the microlens. In addition, the light flux is collected on a photoelectric conversion element disposed at the innermost portion of the sensor.
The problem in the related art will be described with reference to FIG. 11. As described above, recently, a microlens 11 is provided for each pixel on the image sensor surface. The microlenses 11 are provided periodically corresponding to the pixels, and the microlens 11 has a convex phase structure in the air. Further, a photoelectric conversion unit 12 is also provided, corresponding to each pixel. In addition, reference numeral 9 indicates an internal structure (not illustrated) of the image sensor.
The periodical phase structure serves as a reflect-diffraction grating, and some of the imaging light flux incident to the image sensor surface is diffracted and reflected thereon. In this case, zeroth order reflected light causes a failure as stray light. However, particularly, ±1st-order (and higher order) reflect-diffracted light causes a problem.
The reflect-diffracted light is reflected at a different angle from the angle when being incident to the image sensor by diffraction. Since the reflect-diffracted light is obliquely incident to a diachronic coat of an ultra-violet infrared ray (UVIR) cut filter placed just before the image sensor, the diachronic coat has a characteristic different from a cut wavelength assumed when the diachronic coat is designed.
As a result, some of wavelength components of the diffracted light are reflected by the UVIR cut filter and incident onto the image sensor again to become a ghost image. Some of the diffracted light is reflected and diffracted again from the image sensor and repeatedly reflected between the image sensor and the UVIR cut filter, and has an influence as the ghost in a wide range on the image sensor. The reflect-diffraction ghost is shown as a regular red pattern, when a bright spot exists on the image plane.
In order to reduce an amount of light lost on the microlens surface, Japanese Patent Application Laid-Open No. 6-5829 discusses a transparent film having a lower refractive index than the refractive index of the microlens that is installed on the surface of the microlens. However, this cannot improve the above-described reflect-diffraction ghost.
As a countermeasure for preventing the reflect-diffraction ghost, there is considered a method of decreasing an effect as the diffraction grating by determining an optical height of the microlens. In the case of determining the optical height of the microlens, a method of decreasing the height thereof to make the microlens close to a plane. In the case of decreasing the height thereof by changing a curvature, a substantial optical height may be decreased by increasing not only a radius of curvature but also changing a refractive index of a medium in front of and behind a refractive surface.
However, as a result, refractive power of the microlens is weakened, thereby affecting an effect of improving light collecting efficiency, which is an original objective to install the microlens.